The rhg1-b allele of soybean is widely used for resistance against soybean cyst nematode (SCN), the most economically damaging pathogen of soybeans in the United States. Gene silencing showed that genes in a 31-kilobase segment at rhg1-b, encoding an amino acid transporter, an α-SNAP protein, and a WI12 (wound-inducible domain) protein, each contribute to resistance. There is one copy of the 31-kilobase segment per haploid genome in susceptible varieties, but 10 tandem copies are present in an rhg1-b haplotype. Overexpression of the individual genes in roots was ineffective, but overexpression of the genes together conferred enhanced SCN resistance. Hence, SCN resistance mediated by the soybean quantitative trait locus Rhg1 is conferred by copy number variation that increases the expression of a set of dissimilar genes in a repeated multigene segment.

A laser-Doppler velocimeter study of velocity profiles in the laminar boundary layer adjacent to a heated flat plate revealed that the seed particles used for the LDV measurements were driven away from the plate surface by thermophoretic forces. causing a particle-free region within the boundary layer of approximately one half the boundary layer thickness, Measurements of the thickness of this region were compared with particle trajectories calculated according to several theories for the thermophoretic force. It was found that the theory of Brock, with an improved value for the thermal slip coefficient, gave the best a~reement with experiment for low Knudsen numbers, ~/R = O(lo-). where ~ is the mean free path and R the particle radius.Data obtained by other experimenters over a wider ranqe of Knudsen numbers are compared, and a fitting formula for the thermophoretic force useful over the entire range 0 ~ ~/R ~ oo is proposed which agrees within 20-25% with the majority of the available data.

No abstract